There are a number of coherent imaging modalities utilizing electronic beamformation to effect pulse-echo or energy-reflection imaging, in particular radar imaging, ultrasonic imaging and sonar imaging. In many applications, especially real-time medical ultrasonic imaging, it is important to minimize the time necessary to acquire each image (i.e. the time necessary to scan a given field of view) in order to attain a high frame rate.
The requirement to scan a field of view rapidly is always moderated, however, by the need to maintain adequately fine spacing of the beams used to illuminate the field of view and to acquire the image. The spacing of these beams defines an azimuthal sampling grid, referred to as scan lines, and it is well known that the information in the image can be accurately preserved only if this grid is finer than a specific sampling limit in accordance with sampling theorems for one or more dimensions. Prior art systems have often compromised image quality in favor of frame rate by undersampling the field of view. The visible artifact associated with undersampling is shift variance, characterized by sensitivity of the image field to small shifts of the sampling grid with respect to the underlying object field; in an ideal imaging system, the image field has no sensitivity to the positioning of the sampling grid on the object field.
Adequate sampling is made more difficult to achieve by the requirement that the displayed image field consist of detected samples, typically log-magnitude detected samples, although phase detection has also been used in the prior art. Hayes has shown in "The Reconstruction of a Multidimensional Sequence from the Phase or Magnitude of Its Fourier Transform," IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-30, No. 2, April 1982, pgs. 140-154, that, for cases of interest, an image must be oversampled by a factor of two in each of range and azimuth for all information to be preserved through a process of magnitude detection or phase detection.
The need to scan a field of view rapidly while maintaining adequate line density has been addressed in the prior art with multiple beam techniques, where two or more independent receive beams are simultaneously formed to detect the echoes from one or more simultaneously excited independent transmit beams. An example is O'Donnell U.S. Pat. No. 4,886,069, entitled Method Of And Apparatus For Obtaining A Plurality Of Different Return Energy Imaging Beams Responsive to A Single Excitation Event, issued Dec. 12, 1989, in which multiple receive beams are used in conjunction with one transmit beam. Another multiple beam technique is disclosed in Drukarev, et al., U.S. Pat. No. 5,105,814, entitled Method Of Transforming A Multi-Beam Ultrasonic Image, issued Apr. 21, 1992, in which multiple non-colinear receive beams are formed to align one-for-one with the same multiplicity of transmit beams.
A similar scheme for acquiring three-dimensional images using two-dimensional arrays is disclosed in S. Smith, H. Pavy and O. von Ramm, "High-Speed Ultrasound Volumetric Imaging System-Part I: Transducer Design and Beam Steering" IEEE Transactions on Ultrasonics, Ferro-Electrics, and Frequency Control, Vol. 38, No. 2, March 1991, pages 100-108 and in O. von Ramm, S. Smith, and H. Pavy, "High-Speed Ultrasound Volumetric Imaging System-Part II: Parallel Processing and Image Display", IEEE Transactions on Ultrasonics, Ferro-Electrics, and Frequency Control, Vol 38, No. 2, March 1991, pages 109-115. For each transmit beam, which illuminates many points, eight simultaneous receive beams are formed.
All of these prior art techniques may reduce the time necessary to scan a field of view, but they can result in a degradation of image quality due to the deliberate misalignment of transmit and receive beams and/or due to interbeam interference when multiple transmit beams are used. The degradation is systematically manifested as shift variance. In the first case, that is because the resulting two-way beams do not generally traverse a straight path (resulting in a position-dependent geometric distortion). In the second case, that is because the resulting two-way beams are not uniform from beam to beam. These artifacts are apparent in systems that utilize focusing for near-field imaging, and they are generally unacceptable in high resolution medical ultrasonic imaging in particular.